1. Field of the Invention
The invention relates to control systems for automatically providing electrical power to lighting loads at night and automatically shutting off the power in daytime, by the use of light-sensitive photo cells or the like together with appropriate switches, relays or the like.
2. Description of the Prior Art
One type of prior art photo control system for a lighting load, in general consisted of a photo cell or the like light-sensitive device which operated an on-off switch in accordance with ambient light intensity, so as to provide power to the lighting load at night and to shut off the power supply during daylight hours. Such a system may or may not also use a timing device, such as a clock, for turn-off during the night.
Photo controls used in street lighting applications generally have two basic components, to wit, a photosensitive element such as a photo cell and a relay. A photo cell is a solid state device which typically consists of a semiconductive layer of cadmium sulfide and terminals. The nature of this cell is to have a high ohmic resistance in the dark (during the night), and a low ohmic resistance in the light (during the day). When the resistance is high the relay is closed; conversely, when the resistance is low the relay is open. A photo cell that is electrically energized will age with time, resulting in a gradual increase of its resistance at a given light level. Therefore, the aging of an electrically energized photo cell has the effect of increasing the light level at which the photo control turns off the lighting load. After many years of service a photo cell "sees" no light, causing the relay to remain closed. In this condition, the photo control will keep the street light on continually.
In addition, the relay also can fail, generally in two ways. The first way is by a break in its winding, and the second is by deterioration and eventual final welding shut of its contacts. Both failure modes will cause the light to be energized continually. The percentage of relay failure is generally not as great as that of photo cell failures. In any event, a normal photo control eventually will fail in an on mode by the nature of its construction.
The need for a fail-off photo control, i.e., a light-sensitive control circuit for a night lighting load which, when a component such as the photo cell, per se, or parts of the relay fail to function properly, will permanently shut off the power supply to the load, has arisen because of the energy crisis. Previous to the energy crisis, it was considered desirable to have street-lights or other night lighting loads remain on when photo control failure occurred. With low energy costs, it was justifiable to have the assurance that a street-light would always be lit at night, even though it also would be on all day. As energy costs continue their upward rise, this justification has become less economically sound. A fail-off photo control, i.e., one which would fail in such a manner that would cause a street-light to remain off, will conserve energy which would be wasted during daytime lamp operation. This energy saving is at the expense of having a few street-lights out at night.